For example, a welding robot is a six-axis vertical articulated industrial robot, in which the tip of an arm is provided with a welding tool formed by a welding torch or the like. A joint unit, which is a driving unit of the welding robot, includes a motor serving as a driving source, and a speed reducer configured to transmit a driving force of the motor to the arm. The motor in the joint unit includes a position detector, such as an encoder, that detects the rotational position of the motor.
To the welding robot having the joint unit, a position command signal indicating a rotational position of the motor is given from the outside. Then, the rotation of the motor is controlled such that motor position information detected by the position detector matches a commanded position indicated by the position command signal. The rotation of the motor under such control is transmitted through the speed reducer to the arm, so that the welding torch or the like at the tip of the arm moves to a desired position.
As is well known, a speed reducer formed by combining a plurality of gears has backlash, which contributes to a positional deviation of an arm. An attempt has been made to eliminate the positional deviation of the arm. Specifically, the attempt involves obtaining the amount of backlash in the speed reducer in advance, adding a correction signal for compensating for the amount of backlash to a position command signal so as to generate a position command correction signal, and giving the position command correction signal to the welding robot to eliminate the positional deviation of the arm.
As means of eliminating the positional deviation of the arm, there are techniques disclosed in Patent Literatures (PTLs) 1 and 2 listed below.
PTL 1 discloses a robot control device that includes a position control unit and a backlash correcting unit. The position control unit is a servo mechanism configured to compare a command position signal with a position signal which is an output signal of position detecting means, and perform control such that a tip of an arm is located at a predetermined position specified by the command position signal. After each arm is moved to a predetermined position and a motor rotates by more than or equal to a predetermined amount of backlash in a speed reducer in the same direction as that in positioning for associating the position signal of the position detecting means with the position of the arm, if reversal of the rotation direction of the motor is detected, the backlash correcting unit performs backlash correction which involves adding the amount of correction corresponding to the amount of backlash to the command position signal, and thus eliminates the effect of backlash.
This control device is described as being capable of always performing appropriate backlash correction, and causing the hand tip position of the robot to accurately follow the commanded position.
PTL 2 discloses a numerical control device that includes control direction detecting means for monitoring a position command value for a control object, and detecting a control direction for controlling the control object; correction command value updating means for updating a backlash correction command value depending on the time elapsed after the control direction detected by the control direction detecting means is reversed; and control means for adding the backlash correction command value updated by the correction command value updating means to the position command value, and controlling the rotation of a motor that drives the control object on the basis of the result of the addition.
This numerical control device is described as being capable of compensating for backlash and accurately compensating for elastic deformation that occurs when the direction of movement of the control object is reversed.